Tapered waveguide transition sections



March 24, 1964 s. E. MILLE-R 3,125,517

TAPERED wAvEGuIDE TRANSITION sEcTIoNs Filed Sept. 16, 1957 A 7' TORNEV United States Patent Ofifice 3,126,517 Patented Mar. 24, 1964 3,126,517 TAPERED WAVEGUIDE TRANSITEON SECTEONS Stewart E. Milier, Middletown, NJ., assignor to Bell Telephone Laboratories, Incorporated, New York, NX., a corporation lof New York Fitted Sept. 16, 1957, Ser. No. 683,990 5 Claims. (Cl. 3313-34) This invention relates to waveguide transition sections to connect waveguides of different dimensions for the preferential transmission of the TEM circular electric mode combining low mode conversion, and non-circular mode filtering properties.

In the transmission of electromagnetic wave energy through a hollow conductive pipe or other waveguide, it is well known that the energy can propagate in one or more transmission modes, or characteristic field configurations, depending upon the operating frequency, and that the larger the cross-section of the guide is made, the greater is the number of modes in which the energy can propagate at a given operating frequency. Very general- 1y it is desired to confine propagation of the energy to one particular mode, chosen because its propagation characteristics are favorable for the particular application involved. If the desired mode happens to be the so-called dominant mode, it is feasible to restrict the cross-sectional dimensions of the guide so that no modes other than the dominant mode can be sustained therein. This expedient is not available, however, if the desired mode is not the dominant mode or if a guide of large cross-section is prescribed in order, for example, that advantage may be taken of its relatively low attenuation. This is particularly true of systems employing the TEM circular electric mode. As is well known, the propagation of microwave energy in the form of the TEM mode in circular waveguides is ideally suited for the long distance transmission of high frequency wide band signals since the attenuation characteristic of this transmission mode, unlike that of other modes, decreases with frequency. However, since the TEM mode is not the dominant mode supported in a circular waveguide, energy may be lost to other modes that are also capable of transmission therein.

In an ideal waveguide which is perfectly straight, uniform and conducting, the propagation of T1501 waves therethrough is undisturbed, but slight imperfections in the guide and especially curvature of the waveguide axis may excite waves of other modes and produce serious losses. These losses are attributed mainly to the fact that the bending of the guide produces a coupling between the desired TEM mode and other transmission modes, mainly the TMH mode.

A second class of spurious modes consisting of higher order circular electric modes such as the TEM, TE03, etc., are generated whenever the waveguide diameter is changed. Diameter changes will occur often in any practical system in order to take maximum advantage of the transmission characteristics of the TEM mode. For example, it is known that the transmission loss for the circular electric mode is inversely related to the guide diameter. Hence, long uninterrupted runs of waveguide will be made with large diameter pipe. Multiplexing of a series of frequency bands into the pipe, however, is most efficiently done at certain smaller diameters. Thus, numerous changes in guide size will be made at repeater stations where the various frequency-bands are to be coupled into and out of the system, Similarly, sharp intentional bends can be more easilynegotiated at smaller diameters, which will necessitate further diameter changes wherever such bends are required.

The presence of both classes of spurious modes represents a loss of power in the desired mode and a potential source of distortion. Ideally, a system would be organized to prevent the generation of these spurious modes, but as a practical matter this is not feasible. Thus, it is necessary to provide means for filtering the signal occasionally and for removing any spurious modes that may have been generated elsewhere in the system.

Whereas filtering of the non-circular electric spurious modes without affecting the TEM mode may be readily accomplished, no simple means is known to suppress the higher order circular electric modes without affecting the lowest order wave. Present practice is, therefore, to provide two separate devices, one consisting of a tapered transition section to couple the dissimilar waveguides, where such transition sections are designed to have low mode conversion, and a separate mode filter to reduce the non-circular spurious modes. Obviously, it would be highly desirable to combine both functions into a single apparatus.

It is, therefore, an object of the present invention to translate circular electric wave energy from a waveguide of larger diameter to a second waveguide of smaller diameter with minimum mode conversion to higher order circular electric modes, and simultaneously to filter spurious non-circular electric modes generated elsewhere in the system.

It is a further object of this invention to perform such translation and such filtering over a wide range of frequencies.

In the copending application of J. R. Pierce, Serial No. 416,315, filed March 15, 1954, now United States Patent 2,848,695, issued August 19, 1958, it is disclosed that a helical conductor of diameter greater than 1.2 free space wavelengths will propagate a properly excited circular electric TEM mode with a different phase constant than the TMH mode. This provides a substantial decoupling between these modes, and is in addition, a structure that is easily and economically wound in sections of arbitrary length and shape. It is further disclosed in my copending application, Serial No. 416,316, filed March 15, 1954, now US. Patent 2,848,696, issued August 19, 1958, that if the respective T E01 and TMll modes propagating along the resulting helical transmission path are exposed in a special way to electrically dissipative or lossy material, they will have substantially different attenuation constants. It is further shown that if the difference between the attenuation constants is large, the coupling between the TEM and TMm modes may be made arbitrarily small, resulting in a minimum of degeneration of the TEM mode, without however, any substantial amount of energy being actually lost in the dissipative material.

It is now proposed to utilize these properties in a different manner. It is proposed that the relatively high attenuation constant of the helical guide encased in dissipative material be used not to prevent the conversion of energy into the TMm mode but rather to attenuate such TMll mode energy as may already exist in the propagating energy without in any way affecting the desired mode. By so doing, a simple mode filter is available for eliminating the non-circular electric wave energy.

In the specific illustrative embodiment of the invention to be described in detail hereinafter, a conical tapered transition section of uniform cone angle was constructed by winding insulated wire on a steel mandrel. The dissipative material comprises a cone-like casing having an inside surface contiguous with the outermost part of the helical conductor. Since the longitudinally flowing current of the TMm mode must pass through the lossy material, the attenuation constant for the TMll mode is substantially increased. On the other hand, the component of circumferential current of the TEM mode follows the helical conductor and is consequently unaffected.

Since the tapered section is required to filter a particular family of undesired modes and to prevent the generation of another family of undesired modes, its minimum length may be determined by either of these requirements, and will vary depending upon the particular application. As a practical matter, however, the minimum length of the section will be determined in most cases by the filtering requirements of the section. In this respect there is a distinct advantage in lforming the lter in the shape of a taper, since the attenuation constant of the helix increases with decreasing diameter. As will be explained in greater detail below, this has the effect of considerably reducing the overall length of the lter. The taper itself may be made in any predetermined shape in accordance with the specifications relating to bandwidth, length, and the maximum tolerable spurious mode level.

It is a feature of the yinvention that the section so constructed will operate satisfactorily over a wide frequency range. It is another feature of the invention that such sections can be made simply and inexpensively.

These and other objects, the nature of the present invention, and its various features and advantages, will appear more fully upon consideration of the Various specific illustrative embodiments shown in the accompanying drawings and analyzed in the followin y detailed description of these drawings.

In the drawings:

FIG. l diagrammatically illustrates a guided microwave communication system employing the circular electric wave and utilizing the taper helical section provided by the present invention;

FIG. 2 illustrates the construction of the tapered helix in accordance with the present invention.

Referring more specifically to FIG. 1, a long distance guided microwave communication system is schematically shown. The system is characterized as long to distinguish it from the short distances found in terminal equipment and to define a system in which the factor of transmission attenuation and distortion becomes relatively important. In such a system, therefore, every eifort must be made to minimize the generation of spurious modes and to suppress, by filtering, such spurious modes as are inadvertently produced.

A typical system comprises a terminal station 11 which may be a transmitter, or if this is an intermediate station, a repeater 11 which is to be connected to a receiver or subsequent repeater comprising station 12. The circular electric TEM mode is the mode in which energy is transmitted between stations and since this mode is not usually produced or utilized directly in the components of a station, transducers 13 and 14 are interposed between stations 11 and 12 and the long distance transmission line connected therebetween. Transducers 13 and 14 may be of any suitable well-known types for converting TEM wave energy to and from a dominant Wave mode configuration.

The transmission line itself connecting the two remote stations is not completely straight along its entire length since, in practical installations, it is substantially impossible to maintain the line along a precisely straight path over a long distance. Intentional bends as represented by bend 19 may also be included in order that the line may follow right of ways or other corners. It is these bends in particular that produce the characteristic moding or degeneration of the TEM into TMll wave power. As noted above, sharp bends as represented by 19 are more easily negotiated at smaller diameters. On the other hand, it is well known that the transmission loss for the circular electric mode is inversely related to the guide diameter. Hence, long, uninterrupted runs of waveguide such as 17 and 21 will be made with large diameter pipe. The helical tapered sections of the present invention are therefore ideally suited to connect guides 17 and 21 with bend 19 and are shown in IFIG. l as sections 1S and 20. In addition to ltering the TMll wave energy generated in bend 19, such smaller amounts as may have been generated by smaller irregularities and imperfections in guides 17 and 21 will also be filtered from the system.

In addition, helical tapered sections will be used in the repeater or transmitter stations and at the receiver stations. Representative of such use are the tapered helical sections 16 and 22 shown connecting guides 15 to 17 and 23 -to 21, respectively.

FIG. 2 shows in detail the tapered helical sections of FIG. 1. Each section comprises a conductor 24 wound in a helix having an internal diameter which varies from d1 to d2 to match the solid waveguides 31 and 32. Conductor 24 may be solid or stranded and may comprise a base metal vsuch as iron or steel plated by a highly conductive material such as copper or silver. Adjacent turns such as 25 and 25 of the helix are electrically insulated from each other, and this may be provided for by a small air gap such as 27 or by a high dielectric constant insulation upon the conductors. The pitch distance of the helix, i.e., the distance between the center of turns 25 and 26, and therefore the pitch angle of the helix, should be as small as consistent with the above-mentioned insulating requirement. This distance in all events must be less than one-quarter wavelength and is preferably such that the gap 27 between adjacent turns is less than the diameter of the conductor 24.

The space between adjacent turns of helix 24, i.e., gap 27, is exposed to electrically dissipative or lossy material. This may be done by enclosing helix 24 in a cone-like casing 28 of material having a high electrical loss. Casing 28 may be made of any suitable plastic or dielectric material, such as polyethylene, in which small particles 29 of resistive material, such as iron dust or carbon black, are suspended. It is not desirable that the material of casing 28 extend into the space between adjacent helix turns and therefore casing 28 preferably has `a smooth internal surface of diameter substantially equal to the outside diameter of helix 24. Casing 28 also serves as a protective and supporting structure for helix 24 and forms a more or less permanent structure.

Casing 28 may then be covered with a non-corrosive conductive shield 30 which serves to protect the tapered section from outside mechanical inuences such as weather, moisture and insects and `from electrical inuences such as stray radiation from `adjacent transmission lines.

It is desirable that the cross-section of helix 24 be maintained Ias nearly circular as possible. This condition may be maintained lby employing as the conductor of helix 24, a spiral of spring steel plated with a highly conductive material. The eiect of the spring lwill maintain the desired circularity. Shield 30 may be wound of either overlapping thin strips or may be made of woven braid.

The inside diameters at the ends of helix 24 are made equal to the corresponding diameters d1 and d2 of the circular guides. Thus, the smallest diameter of the taper must be made greater than the critical or cut-off diameter for the TEM mode -in the circular guide. This cut-olf diameter is equal to 1.22m) where A0 is the wavelength in free space of the longest wave in the transmission band. In practice, however, the guide diameter will be anywhere from 1.25 to 10 times the cut-oft diameter.

The attenuation constants in helical guides for the TM modes are given in an article by S. P. Morgan and l. A. Young, entitled Helix Waveguide, published in the November 1956 edition of the Bell System Technical Journal. It is there shown that as the diameter of the helix is decreased the attenuation constants for the TM modes increase. It is, therefore, apparent that a lter for these modes will be most efcient if placed in that portion of the transmission system having the smallest diameter pipe. In FIG. l there is shown a tapered section 22 connecting guide 21 to guide 23. If the TM lter was independent of the tapered section, it would `appear from the above that any TM filter section would most naturally be placed somewhere in guide 23. In doing so, two possible situations suggest themselves. In

the iirst situation the diameter of guide 23 is such that some of the higher order TM modes are cut oli. If they cannot be supported within guide 23 the location of a iilter in that guide rwill not dissipate them and they 'will be reflected at the tapered section. The presence of these reiected waves will cause `distortion of the signal due to conversionreconversion effects. The second possibility is that guide 23 will be capable of supporting all the TM modes developed within the system. However, because of the reduced diameter, the attenuation of the desired TEM mode is increased and consequently every effort will be made to keep guide 23 as short as possible thus precluding the placement of any mode filters in that portion of the transmission system.

The other alternative is to place the filter on the other side of the tapered section. There, however, the diameter size is such that the length of the filter section is increased considerably, making `for a needlessly large iilter. By constructing the taper in the form of a conical helix in accordance with applicants invention, the smallest practical iilter arrangement is obtained, namely, one that takes advantage of the increased attenuation obtained lat decreased diameter sizes, and one that precedes the reduced portion of the transmission system so as to preclude the possibility of reections and does not unduly increase the TE01 losses. Thus, by combining the taper and iilter as taught by applicant, an extremely useful and economical microwave component is obtained.

In all cases it is understood that the above-described arrangements are illustrative of a small number of the many possible specific embodiments which can present applications of the principles of the invention. Numerous and various other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing `from the spirit and scope of the invention.

What is claimed is:

1. In an electromagnetic wave transmission system vfor transmitting the TEM circular electric mode, means for coupling said mode from a rst section of uniform circular waveguide supportive of said circular electric mode having a iirst diameter to la second section of uniform circular waveguide supportive of said circular electric mode having a second diameter, said means comprising Ia conical tapered section formed by winding an elongated member of conducting material in |a substantially helical form, and a jacket of electrically dissipative material surrounding said tapered section, said helix having the space between adjacent turns thereof exposed to electrically dissipative material.

2. 'I'he combination according to claim .1, Iwherein said dissipative material comprises a conical frustum having an inside surface contiguous lwith the outermost part of the helical conductor.

3. The combination according to claim l, wherein said dissipative material comprises smail particles of resistive material suspended in a dielectric material.

4. :In an electromagnetic wave transmission system a first circular transmission medium having a first diameter and a second circular transmission medium having a second diameter smaller tha-n said first diameter, said media being conductively continuous for all circumferential and for all longitudinal current components of high frequency wave energy conducted along said media, a combination electromagnetic mode iilter and transition section for the preferential transmission of the TEM circular electric mode for coupling said first medium to said second medium comprising a tapered transmission medium, said tapered medium being electrically conductive for direct currents from one end thereof to the other, said tapered medium being electrically dissipative for all longitudinal current components in at least one region in every quarter wavelength -of high Ifrequency wave energy conducted along said tapered medium, said tapered medium being conductively discontinuous at lleast once for all circumferential current components of high frequency wave energy conducted Ialong said tapered medium.

5. In an electromagnetic wave transmission system propagating circular electric and non-circular electric mode wave energy, a waveguide transition section for the preferential transmission of the TEM circular electric mode to connect a iirst section of circular waveguide having a smooth electrically continuous internal Wall surface of a rst diameter to a second section of circular waveguide having a smooth electrically continuous internal Wall surface of a second diameter smaller than said iirst diameter, said transition section comprising an elongated member of conductive material Wound in a substantially helical form with a diameter 4that tapers from said iirst diameter at one end to 4said second diameter at the other end, and means for attenuating said non-circular electric mode comprising an electrically dissipative material surrounding said helix and exposed between adjacent turns of said helix, said helix having an increasing attenuation constant for `said non-circular mode as the diameter of said taper decreases from said iirst diameter to said second diameter.

References Cited in the file of this patent UNITED STATES PATENTS 2,701,861 Andrews Feb. 8, 1955 2,748,350 Miller May 29, 1956 2,770,7'83 Clavier NOV. 13, 1-956 2,848,695 Pierce Aug. 19, '1958 2,848,696 Miller Aug. 19, 1958 FOREIGN PATENTS 1,118,560 France Mar. 19, 1956 

1. IN AN ELECTROMAGNETIC WAVE TRANSMISSION SYSTEM FOR TRANSMITTING THE TE01 CIRCULAR ELECTRIC MODE, MEANS FOR COUPLING SAID MODE FROM A FIRST SECTION OF UNIFORM CIRCULAR WAVEGUIDE SUPPORTIVE OF SAID CIRCULAR ELECTRIC MODE HAVING A SECOND DIAMETER, SAID MEANS COMPRISING A CONICAL TAPERED SECTION FORMED BY WINDING AN ELONGATED MEMBER OF CONDUCTING MATERIAL IN A SUBSTANTIALLY HELICAL FORM, AND A JACKET OF ELECTRICALLY DISSIPATIVE MATERIAL SURROUNDING SAID TAPERED SECTION, SAID HELIX HAVING THE SPACE BETWEEN ADJACENT TURNS THEREOF EXPOSED TO ELECTRICALLY DISSIPATIVE MATERIAL. 